This invention relates to formulations of biologically active agents suitable for agricultural field application.
Agricultural chemicals, particularly herbicides, are sold in a wide variety of formulations, including solid formulations such as powders, dusts, granules and time-release microcapsules, liquid formulations such as solutions and emulsions, and suspensions of solids in liquid carriers. The choice of the formulation is generally governed by such considerations as the physical characteristics of the active ingredient, the type of crop or weed species to which the formulation is to be applied and its growth cycle, and the timing of the application (postemergence or preemergence).
Delayed-release formulations are favored for their ability to provide herbicidal efficacy over an extended period of time. Initial activity is often low, however, while the concentration of active ingredient in the soil slowly rises to an effective level. In many cases, delayed-release formulations must be supplemented with a formulation providing immediate delivery of the same active ingredient to both avoid the initial weed growth and provide continuous control over the crop's growth cycle. The conventional solution is a two-package formulation in which the delayed-release and immediate delivery formulations are kept apart until field application.
In many cases, two different active ingredients are needed at the same time for effective weed control. The combination of two active ingredients in a single formulation, however, is not readily achieved in many cases. Difficulties such as chemical or physical incompatibility of the active species, and the need for special formulating techniques for certain species due to low melting points or other characteristics frequently make it impractical or unfeasible to combine the two species in a single-package formulation.
Adding further to these difficulties is the need to maintain the physical stability of the formulation during storage. This is often critical to its effectiveness when finally applied to a field for crop protection. This is of particular concern with regard to multi-phase systems such as suspensions and emulsions. Immediate delivery systems such as emulsions, emulsifiable concentrates and suspensions of solid particles can undergo transformations such as changes in phase, particle settling and particle or droplet agglomeration. Such transformations are often detrimental to the dispersibility of the active ingredient in the field, and sometimes even to the ability of the formulation to deliver the active ingredient to the soil or plant surface to which the formulation is applied. Furthermore, such transformations are difficult if not impossible to reverse.
In the case of water-suspended time-release microcapsules, it is essential that the active ingredient remain in the microcapsule until field application. Since the encapsulated phase is generally a solution of the active ingredient in a non-water-miscible solvent, the surrounding water serves as a barrier preventing rapid outward diffusion of the active ingredient. When the solvent is a volatile solvent, the water also serves to prevent evaporation of the solvent and any solidification of the active ingredient which might occur as a result.
The placement of an additional dispersed phase, whether solid or liquid, in the water presents a risk of upsetting the stability of a multi-phase formulation. Adding solid particles or suspended droplets to a microcapsule suspension should be particularly hazardous to the suspension stability of the entire system, since the surface of the microcapsule is chemically distinct from that of the added material. The microcapsule generally has a microporous polymer shell whose surface charges and accessible functional groups will differ considerably from those of the active ingredient which is present on the droplet or particle surface. The introduction of the droplet or particle may thus upset the balance of surface forces set up by the suspension agents normally used for the microcapsules and interfere with their effectiveness.
In addition to maintaining the microcapsule dispersion, the system must hold the added phase in a dispersed state as well, and also maintain the water barrier around the microcapsules. Thus, each dispersed phase must be prevented from agglomerating with itself as well as with the other dispersed phase. In view of the surface differences between the two dispersed phases, it is not likely that a single suspension system would be readily found which would serve all these functions.
When the material added to the microcapsule suspension are droplets forming an emulsion, it is often critical that the active ingredient in the emulsion not crystallize during storage. For active ingredients with low water solubility, crystallization is avoided by careful maintenance of the equilibrium distribution of the active between the droplets and the water, and control of the diffusion of the active across the phase boundary, which involves careful control of the proportion of solvent to active ingredient in the droplets. The introduction of active ingredient to the continuous phase from a second source (i.e., diffusing out of the microcapsule) runs the risk of causing active ingredient to crystallize out of the continuous phase due to an abnormally high concentration.